Generally, an inner diameter, straightness, and cylindricity of a hole of a cylindrical component often used as an automobile component or a machine tool component are measured by a method in which a roundness measuring apparatus or the like is used, an object to be measured (hereinafter referred to as a workpiece) is secured, and a contact detector is rotated along an inner surface of the hole and moved in the longitudinal direction of the hole for measurement. For a small workpiece, a roundness measuring apparatus which rotates the workpiece for measurement is used.
However, these roundness measuring apparatuses are general-purpose measuring apparatuses, which have various functions and are expensive. The roundness measuring apparatuses are of contact type, and a scratch or a trail of a contact may remain on a surface to be measured. Further, if measuring pressure is reduced to prevent a scratch on the surface to be measured, the contact is caught by a burr or a groove, if any, in an inner peripheral portion of the hole, which causes variations in measurement values.
The present invention is made in view of such circumstances, and has an object to provide an inexpensive measuring apparatus which can measure an inner diameter, straightness, and cylindricity of a hole formed in a workpiece without contact.